1. Field of the Invention
The invention relates in general to a display device and a method of manufacturing the same, and more particularly to a light-emitting display device and a method of manufacturing the same.
2. Description of the Related Art
An organic light-emitting display device, which is a self-emissive device, advantageously has the properties of being driven by a low DC voltage, high luminance, high efficiency, high contrast, light weight, reduced thickness, and flexibility, and thus is expected to be in the mainstream of the next generation of flat panel displays.
FIG. 1 is a schematic illustration showing a conventional organic light-emitting display device 100. The organic light-emitting display device 100 includes sub-pixels 110 arranged in a matrix. Each sub-pixel 110 includes a switch element 120 and a light-emitting element 130. The switch element 120 is typically a thin film transistor (TFT) for driving the light-emitting element 130, and the light-emitting element 130 is typically an organic light-emitting diode (OLED). As shown in FIG. 1, the switch element 120 includes a gate electrode 121, a source 122 and a drain 123. The light-emitting element 130 includes a first electrode 131, a second electrode 132 and a light-emitting layer 133. The light-emitting layer 133 could be a single organic film or a multi-layer structure including several organic thin films, depending on the practical application. The first electrode 131 is disposed on the insulating layer 111 and fills a contact hole 111 a for electrically connecting the light-emitting element 130 and the switch element 120. The switch element 120 provides a driving voltage or current to the light-emitting element 130 and controls the light-emitting element 130 to emit light.
In general, the method of manufacturing the display device 100 is mainly divided into processes of manufacturing the switch element 120 and processes of manufacturing the light-emitting element 130. During the manufacturing steps, especially in the processes of manufacturing the switch element 120, it is inevitable that undesired particles, such as dust in unclean air or processing machines, or impurities in the materials, contaminate the display device. If a single undesired particle becomes enclosed by any layer of the display device, the layers subsequently formed above the particles will be uneven, and the formation of abnormal protrusions could cause a short circuit. Besides, if undesired particles attach to the light-emitting area (i.e. the pixel area), it could cause image defects due to light scattering or light absorption by the undesired particles, and the production yield of display devices would therefore be reduced.
FIG. 2A˜FIG. 2C schematically illustrate a defect in the process of manufacturing the conventional display device 100. Firstly referring to FIG. 2A, a substrate 140 is provided with a buffer layer 141 on its upper surface 140a. Next referring to FIG. 2B, a switch element 120 comprising a gate electrode 121, source 122, and drain 123 is formed. In the process of manufacturing the switch element 120, it is assumed that particles 191 are allowed accidentally to fall on and attached to the films. Afterward, a light-emitting element 130 comprising a first electrode 131, a light-emitting layer 133 and a second electrode 132 is formed, as shown in FIG. 2C. Also, the first electrode 131 fills in the contact hole 11 la for electrically connecting the light-emitting element 130 and the switch element 120. When one of the particles 191 has fallen on an area to become a light-emitting area P, it causes a protrusion to be formed in the first electrode 131 overlaying it, possibly leading the first electrode 131 and the second electrode 132 to be short-circuited as shown in the figure.
Besides the undesirable particles, grain growth during the silicon crystallization could cause defects to occur in a display device. FIG. 3 is a schematic illustration showing another possible defect in the conventional display device 100. Since the trend of display devices has been to build logic circuits and memory circuits in a display substrate, a low temperature polysilicon (LTPS) manufacturing process has been developed in consideration of the demands of limitations on heat-resistance of the substrate, high integration and high carrier mobility. The polycrystallization methods of the LTPS manufacturing process mainly include an excimer laser annealing (ELA) process or a metal induced crystallization (MIC) process. In addition to the problem of the particle adhesion in the manufacturing processes, the display device 100 manufactured by the LTPS process further encounters the problem of the residue on the active layer 124. With regard to the excimer laser annealing method applied in the process of the crystallization of the active layer 124, the grains at the grain boundary may jostle against the others and be ridged due to the stress in their own growth and that of neighboring grains. After etching the active layer 124, the protrusions 150 due to active layer residues can cause roughness on top of the buffer layer 141. In addition, with regard to the metal-induced crystallization method, with nickel di-silicide (NiSi2) trapped at a grain boundary, the protrusions 150 could also be formed and cause an unsmooth surface of the buffer layer 141. Similarly, after a subsequent evaporation process in the formation of the light-emitting element 130 is finished, the first electrode 131 and the second electrode 132 may become short-circuited due to a series of laminated protrusions.
As mentioned hereinabove, both the polysilicon process and the amorphous silicon process may encounter the problem of particle generation in the processes of manufacturing the display device. The low temperature polysilicon manufacturing process may have a further problem of an unsmooth surface (for example, protrusions 150) of the buffer layer 141. A new display device structure provided according to the invention is able to prevent the above-mentioned problems in the light-emitting or pixel area.